In the pollution monitoring industry it is often necessary to create blends of certain pollutants in carrier gases in known quantities for use as calibration mixtures for calibrating pollution monitors. Similarly, in the semiconductor industry it is often necessary to create blends of dopants at very dilute ratios in carrier gases for the doping of semiconductors in reactors. These very dilute blends, in parts per million or fractions of a part per million, are not sufficiently stable to be stored in compressed gas cylinders as pre-mixed blends; therefore, they must be created dynamically at the time of use. In one conventional method, a high ratio blend of the material is stored in a cylinder with a carrier gas. At the time of use this relatively concentrated blend is diluted with a much larger flow rate of carrier gas, using conventional means for measuring and controlling the flow rates of both the mixed gas and the added diluent. In another conventional method, a permeation tube or like source is used to store a small amount of the pure material. The permeation source emits this material at a consistent flow rate at a given temperature. The permeation source is contained in an oven to control the temperature, and the emission rate (typically a few hundred nanograms per minute) is measured by successive weighings. A carrier gas of known flow rate is passed over the permeation source, thereby delivering a known blend in the sub-ppm range. See, in this regard, O'Keeffe U.S. Pat. No. 3,412,935.
The first method described above is somewhat cumbersome because of the large storage cylinders, and the need to control two flow rates to give the required dilution ratio. A further problem for precision work is the unreliability of the mixture stored in the pressure cylinder. The second method described above works well except for a requirement for very precise temperature control. The typical O'Keeffe permeation source is a tube having an emission rate which varies approximately 10 to 15 percent per degree centigrade. This permeation source must therefore be maintained in an oven with an extremely precise temperature control and requires from 5 to 24 hours for stabilization after the oven is activated following a period of storage or transportation without oven control. In a newer version of a permeation source, devised by R. Chand, temperature dependence is reduced to about four percent per degree centigrade for liquid-phase devices and to about 1.0 percent per degree centigrade for gas-phase devices. Such improved devices, therefore, are less subject to temperature variations, but they still require at least modest temperature control when utilized in precision applications.
The present invention utilizes a permeation source, but overcomes the foregoing drawbacks by controlling the rate of delivery of the carrier gas without exercising control over the rate of emission from the permeation source. Carrier gas flow is automatically regulated by an electronic mass flow controller in accordance with temperature variations of the permeation source to compensate for temperature dependent changes and emission from the permeation source, thereby maintaining the desired proportionality of the blend. Accordingly, no temperature-controlled oven is needed and fixed, known, dilute gaseous blends are delivered upon command.
In a specific embodiment, automatic regulation of the mass flow controller is obtained by an electrical signal circuit including a thermistor closely associated with the permeation source. The thermistor has a negative temperature coefficient of resistance which is adjustable by a voltage divider to compensate for the positive temperature coefficient of emission of the permeation source.
Prior art, additional to the foregoing, includes U.S. Pat. Nos. 1,719,898 and 3,702,619.